EP2S180F1020C4 Datasheet, PDF (745/768 Pages) Altera Corporation – Stratix II Device Handbook, Volume 1

High-Speed Board Layout Guidelines
Figure 11–9 shows the propagation delay versus the dielectric constant
for microstrip and stripline traces. As the dielectric constant increases, the
propagation delay also increases.
Figure 11–9. Propagation Delay Versus Dielectric Constant for Microstrip & Stripline Traces
300
250
200
tPD (ps/inch) 150
100
50
Microstrip
Stripline
T = 1.4
Z₀ = 50 Ω
W_stripline = 9.0 mils
W_microstrip = 8.0 mils
0
1
2
3
4
5
6
7
8
9
ε_r
Altera Corporation
May 2007
Pre-Emphasis
Typical transmission media like copper trace and coaxial cable have
low-pass characteristics, so they attenuate higher frequencies more than
lower frequencies. A typical digital signal that approximates a square
wave contains high frequencies near the switching region and low
frequencies in the constant region. When this signal travels through
low-pass media, its higher frequencies are attenuated more than the
lower frequencies, resulting in increased signal rise times. Consequently,
the eye opening narrows and the probability of error increases.
The high-frequency content of a signal is also degraded by what is called
the “skin effect.” The cause of skin effect is the high-frequency current
that flows primarily on the surface (skin) of a conductor. The changing
current distribution causes the resistance to increase as a function of
frequency.
You can use pre-emphasis to compensate for the skin effect. By Fourier
analysis, a square wave signal contains an infinite number of frequencies.
The high frequencies are located in the low-to-high and high-to-low
transition regions and the low frequencies are located in the flat (constant)
regions. Increasing the signal’s amplitude near the transition region
emphasizes higher frequencies more than the lower frequencies. When
this pre-emphasized signal passes through low-pass media, it will come
out with minimal distortion, if you apply the correct amount of
pre-emphasis (see Figure 11–10).
11–9
Stratix II Device Handbook, Volume 2